Because it works. Done well, first person narrative can be revelatory, powerful and deliciously angsty. Done badly it descends into a mass of tired fandom cliches which make you wonder what sedative the writer was on, and will it help you sleep as well as their fic does?
As soon as 'I' comes to town in a fic it's make or break time.
The introspective stream of consciousness fic has been ill-treated. The ones where "I sit and polish my P-90, check out Daniel's ass as he's bending over and think whoooaaa, where'd that thought come from. Am I gay - Christ, I'd like to fuck him." Even worse, the clear bold type of JACK or DANIEL at the start of every first person narrative just in case we, the fans, the ones who watch the show and know the characters, don't know who is 'talking'. Hrrmph.
As well as that, such fics rarely skim the surface and don't delve too deeply into the character - dealing mainly with a lot of mawkish 'I'm so sorry's' from both Jack and Daniel while they wonder what went wrong in S4. Which is a goddamn shame, because the pleasure of first person narrative is surely that the character is stripped naked (figuratively, and yes, often literally - since this IS slash) and their messed up psyches are there for the playing with by the writer.
First person stories done really, really disgustingly, envy inducingly well in the fandom include Anna's 'Lost In Translation', Ellison Wonderland's 'Denying The Battle' and Widget's much nommed and delightfully weird 'Bluestoke'. Okay, yeah, I can't resist Widget's 'Jack' any more than I can resist Anna's. All three stories are examples of character voice captured and in Ellison's case, portrayed in a frightening altered state. Then there's Spring's 'Doors' - which while not technically a first person series delves into introspective asides which have the characters nailed. Daniel's little musings on the word 'inane' struck a cord with me as a great example of the kind of things that do float through a linguist's head.
You see? The 'I's DO have it. So why so much lousy writing constructed around this viewpoint?